Preparation of attrition resistant, silica-containing catalysts by the steps of forming a slurry, spray drying the slurry, and calcining and activating the resulting spray-dried microspheres is known in the art. Prior art procedures have involved combining the silica component, in the form of a solution or colloidal sol, with catalyst, catalyst precursor, or catalyst support to form a slurry. The slurry is then spray dried and calcined and activated to form the catalyst. The silica component, which functions as a binder, is normally dispersed throughout the particles.
U.S. Pat. No. 4,677,084 to Bergna discloses an improvement in the process of preparing attrition resistant catalysts, in particular mixed oxide catalysts of vanadium and phosphorus (V/P/O) which are used in the oxidation of hydrocarbons. A solution of oxide precursor, such as polysilicic acid, is used to form an oxide shell over particles of catalyst, catalyst precursor or catalyst support. The process includes the steps of forming a slurry of the catalyst, catalyst precursor, or catalyst support particles with a solute of the oxide precursor (polysilicic acid) having a particle size of not greater than 5 nm, spray drying, and calcining. The amount of solute and particles are selected such that the oxide shell which forms comprises 3-15% by weight of the total weight of particles and oxide. The Bergna patent illustrates that a V/P/O catalyst product prepared using polysilicic acid as the oxide source is superior in terms of attrition resistance to a V/P/O catalyst prepared using colloidal silica as the oxide source.
Although the process and product improvements realized using polysilicic acid (PSA) as oxide source in a V/P/O catalyst in accordance with the Bergna teaching are substantial, a significant problem is associated with the use of polysilicic acid as oxide source. Polysilicic acid is normally made by deionizing a sodium silicate solution. Upon deionizing, the polysilicic acid immediately begins to polymerize and ultimately a gel is formed. When the polysilicic acid solution has gelled, it becomes intractable and cannot be further processed. In practice it has been found that the polysilicic acid is less suitable for processing after a relatively short time, and that preferably a PSA/VPO precursor slurry must be entirely spray dried within 3-4 hours after the PSA solution is generated. This places a significant constraint on the process when it is scaled up to commercial production levels.